Extrusion Processing

The most important processing technique for common thermoplastics is extrusion, where the plastic material is melted in a tubular metal chamber and the melt forced through a die. The design of an extruder is not unlike a toothpaste tube (heated, of course, to melt the resin), and tubular products such as plastic rods, plastic tubes, plastic drinking straws, coatings on electrical wire, and fibers for textile applications can be manufactured using an appropriately engineered die. To exert enough pressure to force the viscous melt through the small die orifice, an Archimedean screw is used. Most of the heat needed to melt the resin is derived from the mechanical shearing action of the screw, although external heating is also provided. The screw transports the resin from the inlet (at the hopper) through a long passage with several heating regions into a heated die. The resin passes through a region of the screw (with decreased depth in screw channels) that ensures further mixing and consolidates the melt removing any empty spaces or bubbles in melt prior to reaching the mold. The passage of melt is controlled by a layer of mesh on its way to the entrance of the die; this breaker plate assembly (with screen pack) serves to filter out any particulate debris and to control the melt flow into the die. The design of the die determines the geometric features of the product extruded. Figure 2.11 shows the main features of a simple single-screw extruder, along with three types of common extrusion dies. The simplest die is a precisely drilled hole or a slit yielding a rod or a ribbon product. A slightly more complicated design (a circular orifice with a central solid region) produces pipes and tubing. The first two dies shown in the diagram are for tube (or pipe) products and laminates. The third is a specialized die for coating thermoplastic resins on electrical conductors. As the conductor is drawn through the cylindrical die, it contacts the molten polymer introduced

Electrical Screen

Electrical Screen

Figure 2.11. Main features of a simple single-screw extruder, along with three types of common extrusion dies.

Figure 2.11. Main features of a simple single-screw extruder, along with three types of common extrusion dies.

from the top of the die. Extremely complicated dies are used in the extrusion of complicated profiles, for instance, in plastic window and door frames.

The product emerging from the die is handled by "down stream" equipment that would essentially cool (in case of pipe cut to size) and collect the product for storage. The actual pieces of equipment used for the purpose depend on the type of product manufactured.

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